The present invention relates to a dispersion in which silica particle agglomerates are dispersed in the form of a colloid, and a process for producing the dispersion. In particular, the present invention relates to a process for producing the dispersion of silica particle agglomerates wherein secondary particles formed by the agglomeration of primary silica particles are dispersed in colloidal form and which dispersion can form a porous coating layer having a high transparency when it is dried.
As for processes for producing dispersions of fine silica particles, many processes for producing colloidal silica wherein the particles are stably monodispersed have been known in the art. For example, U.S. Pat. No. 2,577,484 discloses a process for growing monodispersed colloidal silica particles by treating a diluted aqueous solution of sodium silicate with a cation-exchange resin to form an acidic aqueous active silicic acid solution, adding an alkali to a part of the aqueous solution of active silicic acid to stabilize it, polymerizing it to form a seed dispersion in which the seed particles of silica are monodispersed, polymerizing silicic acid by slowly adding the balance (feed solution) of the aqueous active silicic acid solution while the alkaline condition is kept to polymerize silicic acid and also to grow the monodispersed particles of colloidal silica.
A process most commonly employed for the production of monodispersed colloidal silica on an industrial scale is the above-described ion-exchange resin process. By this process, it has become possible to produce monodispersed colloidal silica having a sharp particle size distribution from an alkali metal silicate such as water glass which is an inexpensive industrial product while the particle diameter is controlled as desired. The monodispersed colloidal silica obtained by such a process is mostly in the form of spherical particles. Because the primary particles are monodispersed without being agglomerated, the particles in dry state are densely packed to form a structure wherein the spaces between the particles are very small. Therefore, the pore volume of the monodispersed colloidal silica is usually smaller than 0.5 ml/g.
On the other hand, a porous silica having a large pore volume include hydrous amorphous silica produced by a wet process. The hydrous amorphous silica is produced by mixing an alkali silicate solution with a mineral acid to form a gel or a precipitate and then wet-pulverizing the product. For example, Japanese Patent unexamined Published Application (hereinafter referred to as xe2x80x9cJ. P. KOKAIxe2x80x9d) No. Sho 55-116613 discloses a process for producing hydrous silicic acid/amorphous silica having a pore volume of at least 0.5 ml/g by adding an acid to an aqueous alkali silicate solution dividedly in two portions, filtering the reaction solution to obtain a wet cake of hydrated silicic acid, applying a shearing force or vibration to the wet cake to form a slurry and spray-drying the slurry. In the silica thus obtained by the wet method, the primary particles are agglomerated to form secondary particles. Thus, the silica keeps a pore volume of as high as at least 0.5 ml/g according to the porosities between the primary particles or secondary particles. However, because the particle diameter is 1 to 20 xcexcm, the scattering degree of light is high and the dispersion is usually milky. Even when a mix coating layer with a binder is prepared, the layer is opaque. Further, because the particles are divided by the pulverization method, the thus-obtained product has a wide particle size distribution and when an aqueous dispersion thereof is left to stand, precipitates are formed and the colloidal properties cannot be obtained.
The wet pulverization of the hydrous amorphous silica by a mechanical means is known as a method for obtaining a colloidal dispersion of fine silica particles. For example, J. P. KOKAI No. Hei 9-286165 discloses a process for producing fine particles of colloidal silica which are secondary particles of 10 to 300 nm which are agglomerates of primary particles of 3 to 40 nm by applying a strong force to synthetic amorphous silica by a mechanical means.
For obtaining an aqueous dispersion of secondary particles of silica, a process wherein silica obtained by a dry method is mechanically pulverized in water is also known. For example, Japanese Patent Publication for Opposition Purpose (hereinafter referred to as xe2x80x9cJ. P. KOKOKUxe2x80x9d) No. Sho 59-169922 discloses a process for providing silica obtained by dry method excellent in dispersibility and transparency by decomposing a volatile silicon compound in flames at a high temperature. Silica obtained by the dry method is in the form of a bulky powder. An aqueous dispersion of this silica is more easily mechanically pulverized than silica obtained by the wet method. However, because the bond between the agglomerated primary particles is relatively weak, problems are caused. Namely, the agglomerated state is easily broken by a strong capillary force applied to the spaces in the course of the drying of water in the formation of the coating layer and, therefore, cracks are easily formed in the coating layer. Furthermore, volatile silicon compounds, mainly silicon tetrachloride, usually used in the dry method have a problem that the cost of the starting materials for them is higher than that of alkali metal silicates such as water glass used also as the starting materials in the wet method.
The object of the present invention is to provide a colloidal dispersion of silica particle agglomerates and a process for producing such a colloidal dispersion. The dispersion of silica particles is composed of fine secondary particles, formed by the agglomeration of primary silica particles, in water to form a colloidal dispersion having a sharp particle size distribution. By drying the dispersion, a porous, highly transparent coating film can be formed.
After intensive investigations, the inventors have found that the above-described dispersion of silica particle agglomerates can be produced by a method which will be described below. Namely, according to the present invention, the above-described dispersion of silica particle agglomerates is obtained by using a colloidal dispersion of silica particle agglomerates having specified physical properties as the seed dispersion and growing the silica particle agglomerates contained in the seed dispersion.
In accordance with the present invention, there is provided a process for producing a colloidal dispersion of silica particle agglomerates having a specific surface area, as determined by nitrogen adsorption method, of 100 m2/g to 400 m2/g, preferably 150 m2/g to 400 m2/g, an average secondary particle diameter of 20 nm to 300 nm and a pore volume of 0.5 ml/g to 2.0 ml/g, which comprises using a colloidal dispersion of silica particle agglomerates having a specific surface area, as determined by nitrogen adsorption method, of 300 m2/g to 1,000 m2/g, and a pore volume of 0.4 ml/g to 2.0 ml/g, preferably 0.5 ml/g to 2.0 ml/g, as a seed dispersion, adding an alkali to the seed dispersion and then adding a feed solution comprising at least one of an aqueous active silicic acid solution and alkoxysilanes to the seed dispersion in small portions, i.e., gradually, to grow silica particle agglomerates. That is, the primary silica particles which constitute the agglomerates become bigger particles at this stage.
There is also provided a process for producing a colloidal dispersion of silica particle agglomerates having a specific surface area, as determined by nitrogen adsorption method, of 100 m2/g to 400 m2/g, preferably 150 m2/g to 400 m2/g, an average secondary particle diameter of 20 nm to 300 nm and a pore volume of 0.5 ml/g to 2.0 ml/g, which comprises using a colloidal dispersion of silica particle agglomerates having a specific surface area, as determined by nitrogen adsorption method, of 300 m2/g to 1,000 m2/g, and a pore volume of 0.4 ml/g to 2.0 ml/g, preferably 0.5 ml/g to 2.0 ml/g, as a seed dispersion, and adding a mixture of a feed solution comprising at least one of an aqueous active silicic acid solution and alkoxysilanes and an alkali to the seed dispersion in small portions or adding the feed solution and the alkali in small portions at the same time to grow silica particle agglomerates. That is, the primary silica particles which constitute the agglomerates become bigger particles at this stage.
There is also provided a colloidal dispersion of silica particle agglomerates having a specific surface area, as determined by nitrogen adsorption method, of 100 m2/g to 400 m2/g, preferably 150 m2/g to 400 m2/g, and a pore volume of 0.5 ml/g to 2.0 ml/g, in which the main peak in the particle size distribution, calculated in terms of weight, is in the range of 10 nm to 70 nm as determined by dynamic light scattering method with a laser granulometer, the standard deviation thereof is not larger than 10 nm, and the cumulative weight of the main peak is at least 80% based on the cumulative total weight of all the peaks.
There is also provided a colloidal dispersion of silica particle agglomerates having a specific surface area, as determined by nitrogen adsorption method, of 100 m2/g to 400 m2/g, preferably 150 m2/g to 400 m2/g, and a pore volume of 0.5 ml/g to 2.0 ml/g, which dispersion has an absorbance of not higher than 0.3 when the solid concentration is 1.0 wt. % and an absorbance of not higher than 0.3 when the solid concentration is 10.0 wt. % at a wave length of 560 nm.
There is also provided a process for producing a colloidal dispersion of porous silica particle agglomerates, which comprises dropping an aqueous solution of active silicic acid in hot water to form a dispersion of agglomerates of silica particles, adding an alkali to the dispersion before the formation of precipitates in the dispersion or before the gelation of the dispersion to stabilize the agglomerates of the silica particles and adding an aqueous active silicic acid solution in small portions while the stable state is kept to grow the silica particle agglomerates. That is, the primary silica particles which constitute the agglomerates become bigger particles at this stage.
There is also provided a process for producing a colloidal dispersion of porous silica particle agglomerates, which comprises heating an aqueous solution of active silicic acid to form a dispersion of agglomerates of silica particles, adding an alkali to the dispersion before the formation of precipitates in the dispersion or before the gelation of the dispersion to stabilize the agglomerates of the silica particles and adding an aqueous active silicic acid solution in small portions while the stable state is kept to grow the silica particle agglomerates. That is, the primary silica particles which constitute the agglomerates become bigger particles at this stage.